The 3rd Generation Partnership Project (3GPP) is responsible for the standardization of the Universal Mobile Telecommunication System (UMTS) and Long Term Evolution (LTE). The 3GPP work on LTE is also referred to as Evolved Universal Terrestrial Radio Access Network (E-UTRAN). LTE is a technology for realizing high-speed packet-based communication that can reach high data rates both in the downlink and in the uplink, and is thought of as a next generation mobile communication system relative to UMTS. In order to support high data rates, LTE allows for a system bandwidth of 20 MHz, or up to 100 Hz when carrier aggregation is employed. LTE is also able to operate in different frequency bands and can operate in at least Frequency Division Duplex (FDD) and Time Division Duplex (TDD) modes.
LTE uses orthogonal frequency-division multiplexing (OFDM) in the downlink and discrete-Fourier-transform-spread (DFT-spread) OFDM in the uplink. The basic LTE physical resource can be seen as a time-frequency grid, as illustrated in FIG. 1, where each resource element corresponds to one subcarrier during one OFDM symbol interval (on a particular antenna port). There is one resource grid per antenna port.
An antenna port is a “virtual” antenna, which is defined by an antenna port-specific reference signal. An antenna port is defined such that the channel over which a symbol on the antenna port is conveyed can be inferred from the channel over which another symbol on the same antenna port is conveyed. The signal corresponding to an antenna port may possibly be transmitted by several physical antennas, which may also be geographically distributed. In other words, an antenna port may be transmitted from one or several transmission points. Conversely, one transmission point may transmit one or several antenna ports. In the following, an antenna port will be interchangeably referred to as an “RS port”.
In the time domain, LTE downlink transmissions are organized into radio frames of 10 ms, each radio frame consisting of ten equally-sized subframes of 1 ms as illustrated in FIG. 2. A subframe is divided into two slots, each of 0.5 ms time duration.
The resource allocation in LTE is described in terms of resource blocks, where a resource block corresponds to one slot in the time domain and 12 contiguous 15 kHz subcarriers in the frequency domain. Two time-consecutive resource blocks represent a resource block pair, which corresponds to the time interval upon which scheduling operates.
Transmissions in LTE are dynamically scheduled in each subframe. The base station transmits downlink assignments/uplink grants to certain UEs via the physical downlink control information (Physical Downlink Control Channels, PDCCH, and enhanced PDCCH, ePDCCH). The PDCCHs are transmitted in the first OFDM symbol(s) in each subframe and spans more or less the whole system bandwidth. A UE that has decoded a downlink assignment, carried by a PDCCH, knows which resource elements in the subframe contain data aimed for the UE. Similarly, upon receiving an uplink grant, the UE knows which time/frequency resources it should transmit upon. In LTE downlink, data is carried by the physical downlink shared data link (PDSCH) and in the uplink the corresponding link is referred to as the physical uplink shared channel (PUSCH).
Demodulation of received data requires estimation of the radio channel, which is performed using reference signals (RS). A reference signal comprises a collection of reference symbols, and these reference symbols and their position in the time-frequency grid are known to the receiver. In LTE, cell-specific reference signals (CRS) are transmitted in all downlink subframes. In addition to assisting downlink channel estimation, they are also used for measurements, e.g. mobility measurements, performed by the UEs. As of Release 10, LTE also supports UE-specific RS aimed for assisting channel estimation for demodulation of the PDSCH, as well as RS for measuring the channel for the purpose of channel state information (CSI) feedback from the UE. The latter are referred to as CSI-RS. CSI-RS are not transmitted in every subframe and they are generally sparser in time and frequency than RS used for demodulation. CSI-RS transmissions may occur every 5th, 10th, 20th, 40th, or 80th subframe according to an RRC configured periodicity parameter and an RRC configured subframe offset.
FIG. 3 illustrates how the mapping of physical control and data channels and reference signals may be done on resource elements within a downlink subframe. In this example, the PDCCHs occupy the first out of three possible OFDM symbols, so in this particular case the mapping of data could start already at the second OFDM symbol. Since the CRS are common to all UEs in the cell, the transmission of CRS cannot be easily adapted to suit the needs of a particular UE. This is in contrast to UE-specific RS, where each UE has RS of its own placed in the data region of FIG. 3 as part of PDSCH.
A UE operating in connected mode may be requested by the base station to report channel state information (CSI), e.g., reporting a suitable rank indicator (RI), one or more precoding matrix indices (PMIS) and a channel quality indicator (CQI). Other types of CSI are also conceivable, including explicit channel feedback and interference covariance feedback. The CSI feedback assists the base station in scheduling, including deciding the subframe and RBs for the transmission, which transmission scheme/precoder to use, and also provides information on a suitable user bit rate for the transmission (link adaptation). A detailed illustration of which resource elements within a resource block pair may potentially be occupied by UE-specific RS and CSI-RS is provided in FIG. 4. The CSI-RS utilizes an orthogonal cover code of length two to overlay two antenna ports on two consecutive REs. As seen, many different CSI-RS pattern are available. For the case of 2 CSI-RS antenna ports we see that there are 20 different patterns within a subframe. The corresponding number of patterns is 10 and 5 for 4 and 8 CSI-RS antenna ports, respectively. For TDD, some additional CSI-RS patterns are available.
Improved support for heterogeneous network operations is part of the ongoing specification of 3GPP LTE Release-10, and further improvements are discussed in the context of new features for Release-11. In heterogeneous networks, a mixture of cells of differently sized and overlapping coverage areas are deployed. One example of such deployments is illustrated in FIG. 5, where pico cells are deployed within the coverage area of a macro cell. Other examples of low power nodes, also referred to as points, in heterogeneous networks are home base stations and relays. The aim of deploying low power nodes such as pico base stations within the macro coverage area is to improve system capacity by means of cell splitting gains as well as to provide users with wide area experience of very high speed data access throughout the network. Heterogeneous deployments are in particular effective for covering traffic hotspots, i.e., small geographical areas with high user densities served by e.g., pico cells, and they represent an alternative deployment to denser macro networks.
A classical way of deploying a network is to let different transmission/reception points form separate cells. That is, the signals transmitted from or received at a point is associated with a cell-id that is different from the cell-id employed for other nearby points. Typically, each point transmits its own unique signals for broadcast (PBCH) and sync signals (PSS, SSS).
The mentioned classical strategy of one cell-id per point is depicted in FIG. 6 for a heterogeneous deployment where a number of low power (pico) points are placed within the coverage area of a higher power macro point. Similar principles apply to classical macro-cellular deployments, where all points have similar output power and may be placed in a more regular fashion than what is the case for a heterogeneous deployment.
An alternative to the classical deployment strategy is to instead let all the UEs within the geographical area outlined by the coverage of the high power macro point be served with signals associated with the same cell-id. In other words, from a UE perspective, the received signals appear to be coming from a single cell. This is illustrated in FIG. 7. Note that only one macro point is shown, other macro points would typically use different cell-ids (corresponding to different cells) unless they are co-located at the same site, corresponding to other sectors of the macro site. In the latter case of several co-located macro points, the same cell-id may be shared across the co-located macro-points and those pico points that correspond to the union of the coverage areas of the macro points. Sync, BCH and control signals are all transmitted from the high power point while data can be transmitted to a UE also from low power points by using shared data transmissions (PDSCH) relying on UE specific RS. Such an approach has benefits for those UEs that are capable of receiving the PDSCH based on UE-specific RS. Those UEs that only support CRS for PDSCH (which is likely to at least include all Release 8/9 UEs for FDD) have to settle for the transmission from the high power point and thus will not benefit in the downlink from the deployment of additional low power points.
The single cell-id approach is geared towards situations in which there is fast backhaul communication between the points associated to the same cell. A typical case would be a base station serving one or more sectors on a macro level as well as having fast fiber connections to remote radio units (RRUs) playing the role of the other points sharing the same cell-id. Those RRUs could represent low power points with one or more antennas each. Another example is when all the points have a similar power class with no single point having more significance than the others. The base station would then handle the signals from all RRUs in a similar manner.
A clear advantage of the shared cell approach compared with the classical one is that the typically involved handover procedure between cells only needs to be invoked on a macro basis. Another important advantage is that interference from CRS is greatly reduced since CRS does not have to be transmitted from every point. There is also much greater flexibility in coordination and scheduling among the points.
The concept of a point is heavily used in conjunction with techniques for coordinated multipoint (CoMP). In the present disclosure, a point (also referred to as a “transmission point” and/or a “reception point”) corresponds to a set of antennas covering essentially the same geographical area in a similar manner. Thus, a point might correspond to one of the sectors at a site, but it may also correspond to a site having one or more antennas all intending to cover a similar geographical area. Often, different points represent different sites. Antennas correspond to different points when they are sufficiently geographically separated and/or have antenna diagrams pointing in sufficiently different directions.
Techniques for CoMP entail introducing dependencies in the scheduling or transmission/reception among different points, in contrast to conventional cellular systems where a point, from a scheduling point of view, is operated more or less independently from the other points. One fundamental property of DL CoMP is the possibility to transmit different signals and/or channels from different geographical locations. One of the principles guiding the design of the LTE system is transparency of the network to the UE. In other words, the UE should be able to demodulate and decode its intended channels without specific knowledge of scheduling assignments for other UEs or network deployments.
For example, different CSI-RS patterns may be transmitted from ports belonging to different transmission points. Feedback based on such patterns may be exploited e.g. for point selection and/or for optimization of precoding weights and CoMP scheduling. Alternatively, the same CSI-RS pattern may be jointly transmitted by different transmission points in order to generate an aggregated feedback including joint spatial information for multiple points. In any case, UEs are generally not aware of the geographical location from which each antenna port is transmitted.
CRS are typically transmitted from a static set of points. Nevertheless, for certain deployments, it is possible to transmit different CRS ports from different geographical locations. One application of this technique is in distributed deployments, where the transmit antennas belonging to the same node are deployed in a non-collocated fashion.
DMRS or UE-specific RS are employed for demodulation of data channels and possibly certain control channels (ePDCCH). Data may be transmitted from different points than other information (e.g. control signaling). This is one of the main drivers behind the use of UE-specific RS, which relieves the UE from having to know many of the properties of the transmission and thus allows flexible transmission schemes to be used from the network side. This is referred to as transmission transparency (with respect to the UE). A problem is, however, that the estimation accuracy of UE-specific RS may not be sufficient in some situations. Furthermore, especially in case of CoMP and/or distributed deployments, the DMRS for a specific UE-might be transmitted from geographically separated ports.
There is a need in the art for mechanisms for improved channel estimation.